Briggs noted that Sanders split the delegates almost evenly with Clinton in Missouri and Illinois, though the former secretary of state won handily in Florida, North Carolina and Ohio.

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"His focus now is on the second half of the delegate selection process. He's looking forward to Arizona, Idaho and Utah next Tuesday and moving on to other important states including Wisconsin, New York and California," Briggs said in an email.

He added of Sanders: "He's taking the contest all the way to the Democratic National Convention in Philadelphia."